In some semiconductor manufacturing processes, a vapour deposition step is necessary in order to deposit matter—organic or inorganic—on a substrate. In some processes, the matter must be deposited within precisely defined areas on the substrate. To simplify the deposition process, a shadow mask is usually applied to one side of the substrate, and cut-outs or openings in the shadow mask define the regions in which matter will be deposited. It is usually a requirement that the material be precisely deposited in the regions corresponding to the openings, so that the boundaries or edges of these regions are sharp. For example, organic material must be deposited in exactly defined regions during the manufacture of organic light-emitting diodes (OLEDs) intended for use in displays or other lighting applications.
However, problems arise when the shadow mask does not maintain a satisfactory close contact with the substrate during the deposition process. Since the substrate and the shadow mask are thin, and have a large area in proportion to their thickness, they tend to sag under their own weight when held in a horizontal position. Material deposition usually takes places in a chamber, usually a vacuum chamber, in which the material to be vapourized is contained in evaporation sources known as ‘boats’ or ‘crucibles’ with or without a connected ‘showerhead’, as will be known to the skilled person. These are heated in some suitable manner, for example electrically, so that the material vaporizes. During this vaporization, high temperatures can be reached in the chamber. As a result, the material of the shadow mask can thermally expand and ultimately part from the substrate. Also, material being deposited does not only arrive at the substrate, but some amount will also arrive at the shadow mask and adhere to it. If the material is being deposited in a vapour deposition process, so that the shadow mask is on the underside of the substrate, superfluous material adhering to the shadow mask in the areas between the openings can be an additional contribution to the shadow mask sag.
When the shadow mask no longer adheres to the substrate over its entire area, the boundaries of the material deposited in the openings are no longer precisely defined, and a poor product quality may be the result. Uneven or smudged edges of deposited regions are unacceptable in a product such as an OLED display, and such poor quality can result in high costs.
A number of approaches have been taken in an effort to reduce the amount of sagging in the shadow mask. For example, the shadow mask can subject to an outward pulling force by means of clamps attached to the substrate or to a frame. In other prior art solutions to the problems associated with material deposition under the adverse conditions mentioned, a metal shadow mask is pre-tensioned prior to being welded onto a mask-retaining metal frame before the vapour deposition commences. However, since material deposits accumulate on the shadow mask, this must eventually be replaced. A shadow mask that is simply attached to the frame by means of clamps can relatively easily be removed and replaced, but a welded shadow mask requires an additional effort in its removal by mechanical means, and the frame surface may also need to be milled before the next shadow mask is welded into place. Therefore, such an approach is relatively expensive. Furthermore, since most OLEDs require multiple layers to be built up in consecutive deposition steps, use of a welded mask may be particularly inconvenient. Other prior art efforts often involve restraining the shadow mask using a spring-loaded holding means to ‘pull’ the shadow mask in an outward direction. The application of a tensile force to the shadow mask aims to prevent ‘folds’ or ‘ripples’ from appearing in the shadow mask when this is subject to thermal expansion during the vapour deposition process. However, a shadow mask used in the manufacture of OLEDs for lighting purposes generally has a high proportion of openings in its overall surface area, also termed ‘negative area’, so that the application of a lateral tensile force is of limited advantage. Also, with this approach, it is difficult to ensure that the shadow mask and substrate remain in position relative to one another. It may happen that the shadow mask shifts or moves to one side during transport or deposition, resulting in unacceptable “smudged” material layers. For this reason, in practice, arrangements are used in which the substrate and the shadow mask are held in a vertical position during the material deposition process. However, for products such as displays, in which material is deposited to form OLED pixel regions, any foreign particles that adhere to the substrate—usually by falling onto the substrate—may result in a visible defect. For this reason, in the manufacture of such products, it would be preferable for the substrate to be held such that the material is deposited from below.